(1) Field of the Invention
The present invention relates to a method for efficiently extracting gas by heating and decomposing gas hydrate pellets and a device therefor.
(2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In regions where pipelines are not constructed, a method has been widely employed in which natural gas is artificially liquefied temporarily, and transported as liquefied natural gas (LNG) by dedicated ships or tank trucks. In LNG, gas in a volume approximately 600 times as much as the volume of LNG can be contained by the liquefaction. However, for the liquefaction, the raw material gas is cooled to an ultra-low temperature of −162° C. Hence, the liquefaction requires power for refrigeration, and storage facilities and the like need to have high thermal insulation performances.
Meanwhile, a gas hydrate is a hydrate which is a solid formed by a reaction of a gas with water. In the gas hydrate, the gas is trapped in a cage made of water molecules. When the raw material gas is natural gas, a mixture gas mainly containing methane is trapped, and this gas hydrate is called natural gas hydrate (NGH). NGH maintains a stable state at low temperature and high pressure, and is ordinarily in a decomposition region at normal temperature and normal pressure. Hence, NGH in land areas exists in permafrost zones, and NGH in sea areas exists below the seabed at depths of water deeper than 500 m, where high water pressures are applied.
In NGH, the gas in a volume approximately 160 times as much as the volume of NGH can be contained in the structures. In addition, NGH is known to have such a unique characteristic that NGH decomposes at a relatively low rate under atmospheric pressure and at temperatures of −10° C. to −20° C., where NGH is in a decomposition region. In this respect, the following novel natural gas transport method has attracted attention. Specifically, NGH is artificially produced, for example, at a pressure of approximately about 5 MPa and a temperature of about 5° C. Then, the NGH is cooled and depressurized, and the hydrate is stored and transported by utilizing the mild region where decomposition can be suppressed.
The hydrate itself is like powder snow (like fine powder) and bulky, and is rarely used in its original state from the viewpoints of transport efficiency and storability. The hydrate is molded with compression into a given shape and size, and is transported or stored in the form of “pellet-shaped” molded articles having diameters of, for example, 2 cm to 3 cm. Hence, in the use of the gas in the pellets as a raw material or a fuel, the pellets are heated and decomposed, and the generated gas is fed to a destination where the gas is consumed.
Here, an example of a mode of artificial production and storage of NGH is introduced. A fine powdery raw material obtained in a hydrate formation device is compressed into a pellet state by a mold or a paired-roller-type press device provided with recessed portions on surfaces thereof, and cooled to a storage temperature. The pellets have enough strength to resist destruction and collapse and to keep their shapes, even when being supplied into a large storage tank having a diameter of 30 m and a height of 60 m, for example. Hence, extraction of the gas by decomposing such firm pellets into water and gas additionally requires an efficient “regasification step.”
An example of the regasification step is shown in Japanese patent application Kokai publication No. 2001-279281. According to this Document, the regasification step is configured as follows. Specifically, pellets are introduced into hot water in a horizontal and rotatable gasification container. The generated gas and water are introduced into a gas-liquid separator, and separated from each other. The gas is extracted from the gas-liquid separator, whereas the water is extracted by a pump, and returned to the gasification container after heating.
Meanwhile, Japanese patent application Kokai publication No. 2005-239782-proposes the following device. Specifically, a ring-shaped nozzle for supplying gas hydrate is disposed at an upper portion in a vertically long gasification container; a rotation shaft provided with a rotatable impeller at a lower end thereof and with an impeller for grinding at an upper portion thereof is disposed at the center in the container; a thick cylindrical heat exchanger is formed around the impeller for stirring; and a bubble separation plate is provided at a bottom portion of the container.